[Physics] Quantum Computing, Qubit Creation/Entanglement

Question

I am currently a high school student researching quantum computing. I was referred to this site by Google and a friend. Currently I am researching the qubit part of quantum computing. My question is exactly how are qubits created in the lab, and how are they entangled? I don't expect the answers to be incredibly specific but a general overview would be of a great deal of help.

Answer 1

Take a proton (the nucleus of Hydrogen - everywhere in water) which has a spin, and since it's charged, it has North and South poles. If you measure it, the North pole points either up or down in your instrument. If you embed it in a magnetic field, it will want to line up with that field, but it can't easily because it's spinning like a little gyroscope, so it precesses like a top. The rate at which it precesses depends on the strength of the field, and that can be detected, and so you have Nuclear Magnetic Resonance, used everywhere in MRI machines.

By manipulating the field, you can put the proton into a state where it's "in-between" up and down. If you measure it, it will be either one or the other, but before you measure it, it's in a mixture of states, called a "superposition".

If you have some number of them, like for example four, by manipulating the field, you can put them all in a mixture state. But it's not like four independent mixtures. Rather it's one mixture of 16 possible states. If you measure all of them at once, you could get any one of the 16 possible answers.

Each one of those states in the superposition is a fully-specified combination of bits, so it's like having 16 different 4-bit computers running in parallel, but they're all running the same program at the same time. The "program" consists of magnetic pulse trains that affect all the states at the same time. That's called "quantum parallelism", and you can see that if you can put enough qubits into this superposition where every one of the 2^N combinations is equally likely, you can carry on 2^N computations in parallel.

Then, suppose one of those computations reaches a result that you want to know. You have to get the result by measuring, but that's complicated to explain and may be a bit much for this answer.

P.S. One of the interesting aspects of quantum computation is that it has to be reversible. So if you have an algorithm that you want to execute on a quantum computer, you have to make sure the algorithm can be run in reverse just as well as forward. So for example, if you have a state machine where either state A or state B can transition to state C, it won't work in a quantum computer unless there is some memory of how C was entered, so the state transition can be "un-done".

P.P.S. Let me take another stab at how you get the results out of a quantum computer. The method I'm familiar with is Lov Grover's Search Algorithm, for doing search in an unsorted table. If the table contains M entries, you create a superposition with M states, one of which will "succeed". Since the only way you can get information out is by measuring, what you need to do is adjust the probability amplitudes of the states so that the successful one has a high probability, so when you measure, it is the one you will most likely see. That is done by a manipulation that transfers some of the probability from the unsuccessful states to the successful one. Then the computation is run in reverse back to the beginning, then run forward again, and the probability-transfer operation is done again. This is done several times, until the successful state has nearly all of the probability. It's important not to do it too many times, because it will start having the opposite effect.